diff --git a/core/fxcrt/fx_basic_gcc.cpp b/core/fxcrt/fx_basic_gcc.cpp
index 440f924..c29c995 100644
--- a/core/fxcrt/fx_basic_gcc.cpp
+++ b/core/fxcrt/fx_basic_gcc.cpp
@@ -16,15 +16,25 @@
   if (!str)
     return 0;
 
-  bool neg = std::numeric_limits<IntType>::is_signed && *str == '-';
-  if (neg)
+  // Process the sign.
+  bool neg = *str == '-';
+  if (neg || *str == '+')
     str++;
 
   IntType num = 0;
   while (*str && FXSYS_isDecimalDigit(*str)) {
     IntType val = FXSYS_toDecimalDigit(*str);
-    if (num > (std::numeric_limits<IntType>::max() - val) / 10)
-      break;
+    if (num > (std::numeric_limits<IntType>::max() - val) / 10) {
+      if (neg && std::numeric_limits<IntType>::is_signed) {
+        // Return MIN when the represented number is signed type and is smaller
+        // than the min value.
+        return std::numeric_limits<IntType>::min();
+      } else {
+        // Return MAX when the represented number is signed type and is larger
+        // than the max value, or the number is unsigned type and out of range.
+        return std::numeric_limits<IntType>::max();
+      }
+    }
 
     num = num * 10 + val;
     str++;
diff --git a/core/fxcrt/fx_basic_gcc_unittest.cpp b/core/fxcrt/fx_basic_gcc_unittest.cpp
index 1eac9e3..8d8d2f5 100644
--- a/core/fxcrt/fx_basic_gcc_unittest.cpp
+++ b/core/fxcrt/fx_basic_gcc_unittest.cpp
@@ -10,16 +10,21 @@
   EXPECT_EQ(0, FXSYS_atoi("0"));
   EXPECT_EQ(-1, FXSYS_atoi("-1"));
   EXPECT_EQ(2345, FXSYS_atoi("2345"));
-  EXPECT_EQ(2147483647, FXSYS_atoi("2147483647"));
   EXPECT_EQ(-2147483647, FXSYS_atoi("-2147483647"));
+  // Handle the sign.
+  EXPECT_EQ(-2345, FXSYS_atoi("-2345"));
+  EXPECT_EQ(2345, FXSYS_atoi("+2345"));
+  // The max value.
+  EXPECT_EQ(2147483647, FXSYS_atoi("2147483647"));
+  // The min value.
+  EXPECT_EQ(-2147483648, FXSYS_atoi("-2147483648"));
+  // With invalid char.
   EXPECT_EQ(9, FXSYS_atoi("9x9"));
 
-  // TODO(dsinclair): These are all wacky .....
-  EXPECT_EQ(2147483623, FXSYS_atoi("2147483623423412348"));
-  EXPECT_EQ(214748364, FXSYS_atoi("2147483648"));
-  // The digit is parsed as a positive value, so we end up not being able to
-  // handle the largest possible negative value.
-  EXPECT_EQ(-214748364, FXSYS_atoi("-2147483648"));
+  // Out of range values.
+  EXPECT_EQ(2147483647, FXSYS_atoi("2147483623423412348"));
+  EXPECT_EQ(2147483647, FXSYS_atoi("2147483648"));
+  EXPECT_EQ(-2147483648, FXSYS_atoi("-2147483650"));
 }
 
 TEST(fxcrt, FXSYS_atoi64) {
@@ -27,16 +32,21 @@
   EXPECT_EQ(0, FXSYS_atoi64("0"));
   EXPECT_EQ(-1, FXSYS_atoi64("-1"));
   EXPECT_EQ(2345, FXSYS_atoi64("2345"));
-  EXPECT_EQ(9223372036854775807LL, FXSYS_atoi64("9223372036854775807"));
   EXPECT_EQ(-9223372036854775807LL, FXSYS_atoi64("-9223372036854775807"));
+  // Handle the sign.
+  EXPECT_EQ(-2345, FXSYS_atoi64("-2345"));
+  EXPECT_EQ(2345, FXSYS_atoi64("+2345"));
+  // The max value.
+  EXPECT_EQ(9223372036854775807LL, FXSYS_atoi64("9223372036854775807"));
+  // The min value. Written in -1 format to avoid implicit unsigned warning.
+  EXPECT_EQ(-9223372036854775807LL - 1LL, FXSYS_atoi64("-9223372036854775808"));
+  // With invalid char.
   EXPECT_EQ(9, FXSYS_atoi64("9x9"));
 
-  // TODO(dsinclair): These are all wacky .....
-  EXPECT_EQ(9223372036854712341LL, FXSYS_atoi64("922337203685471234123475807"));
-  EXPECT_EQ(922337203685477580LL, FXSYS_atoi64("9223372036854775808"));
-  // The digit is parsed as a positive value, so we end up not being able to
-  // handle the largest possible negative value.
-  EXPECT_EQ(-922337203685477580LL, FXSYS_atoi64("-9223372036854775808"));
+  // Out of range values.
+  EXPECT_EQ(9223372036854775807LL, FXSYS_atoi64("922337203685471234123475807"));
+  EXPECT_EQ(9223372036854775807LL, FXSYS_atoi64("9223372036854775808"));
+  EXPECT_EQ(-9223372036854775807LL - 1LL, FXSYS_atoi64("-9223372036854775810"));
 }
 
 TEST(fxcrt, FXSYS_wtoi) {
@@ -44,16 +54,17 @@
   EXPECT_EQ(0, FXSYS_wtoi(L"0"));
   EXPECT_EQ(-1, FXSYS_wtoi(L"-1"));
   EXPECT_EQ(2345, FXSYS_wtoi(L"2345"));
-  EXPECT_EQ(2147483647, FXSYS_wtoi(L"2147483647"));
   EXPECT_EQ(-2147483647, FXSYS_wtoi(L"-2147483647"));
+  // The max value.
+  EXPECT_EQ(2147483647, FXSYS_wtoi(L"2147483647"));
+  // The min value.
+  EXPECT_EQ(-2147483648, FXSYS_wtoi(L"-2147483648"));
   EXPECT_EQ(9, FXSYS_wtoi64(L"9x9"));
 
-  // TODO(dsinclair): These are all wacky .....
-  EXPECT_EQ(2147483623, FXSYS_wtoi(L"2147483623423412348"));
-  EXPECT_EQ(214748364, FXSYS_wtoi(L"2147483648"));
-  // The digit is parsed as a positive value, so we end up not being able to
-  // handle the largest possible negative value.
-  EXPECT_EQ(-214748364, FXSYS_wtoi(L"-2147483648"));
+  // Out of range values.
+  EXPECT_EQ(2147483647, FXSYS_wtoi(L"2147483623423412348"));
+  EXPECT_EQ(2147483647, FXSYS_wtoi(L"2147483648"));
+  EXPECT_EQ(-2147483648, FXSYS_wtoi(L"-2147483652"));
 }
 
 TEST(fxcrt, FXSYS_wtoi64) {
@@ -61,28 +72,40 @@
   EXPECT_EQ(0, FXSYS_wtoi64(L"0"));
   EXPECT_EQ(-1, FXSYS_wtoi64(L"-1"));
   EXPECT_EQ(2345, FXSYS_wtoi64(L"2345"));
-  EXPECT_EQ(9223372036854775807LL, FXSYS_wtoi64(L"9223372036854775807"));
   EXPECT_EQ(-9223372036854775807LL, FXSYS_wtoi64(L"-9223372036854775807"));
+  // Handle the sign.
+  EXPECT_EQ(-2345, FXSYS_wtoi64(L"-2345"));
+  EXPECT_EQ(2345, FXSYS_wtoi64(L"+2345"));
+  // The max value.
+  EXPECT_EQ(9223372036854775807LL, FXSYS_wtoi64(L"9223372036854775807"));
+  // The min value.
+  EXPECT_EQ(-9223372036854775807LL - 1LL,
+            FXSYS_wtoi64(L"-9223372036854775808"));
+  // With invalid char.
   EXPECT_EQ(9, FXSYS_wtoi64(L"9x9"));
 
-  // TODO(dsinclair): These are all wacky .....
-  EXPECT_EQ(9223372036854712341LL,
+  // Out of range values.
+  EXPECT_EQ(9223372036854775807LL,
             FXSYS_wtoi64(L"922337203685471234123475807"));
-  EXPECT_EQ(922337203685477580LL, FXSYS_wtoi64(L"9223372036854775808"));
-  // The digit is parsed as a positive value, so we end up not being able to
-  // handle the largest possible negative value.
-  EXPECT_EQ(-922337203685477580LL, FXSYS_wtoi64(L"-9223372036854775808"));
+  EXPECT_EQ(9223372036854775807LL, FXSYS_wtoi64(L"9223372036854775808"));
+  EXPECT_EQ(-9223372036854775807LL - 1LL,
+            FXSYS_wtoi64(L"-9223372036854775810"));
 }
 
 TEST(fxcrt, FXSYS_atoui) {
   EXPECT_EQ(0, FXSYS_atoui(""));
   EXPECT_EQ(0, FXSYS_atoui("0"));
-  EXPECT_EQ(0, FXSYS_atoui("-1"));
+  EXPECT_EQ(4294967295, FXSYS_atoui("-1"));
   EXPECT_EQ(2345, FXSYS_atoui("2345"));
+  // Handle the sign.
+  EXPECT_EQ(4294964951, FXSYS_atoui("-2345"));
+  EXPECT_EQ(2345, FXSYS_atoui("+2345"));
+  // The max value.
   EXPECT_EQ(4294967295, FXSYS_atoui("4294967295"));
   EXPECT_EQ(9, FXSYS_atoui("9x9"));
 
-  // TODO(dsinclair): These are all wacky .....
-  EXPECT_EQ(2147483623, FXSYS_atoi("2147483623423412348"));
-  EXPECT_EQ(429496729, FXSYS_atoi("4294967296"));
+  // Out of range values.
+  EXPECT_EQ(4294967295, FXSYS_atoui("2147483623423412348"));
+  EXPECT_EQ(4294967295, FXSYS_atoui("4294967296"));
+  EXPECT_EQ(4294967295, FXSYS_atoui("-4294967345"));
 }